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'Relay failure modes [OT]'
2000\03\29@093105 by Roland Andrag

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Hello everyone,

I would like to hear some comments on PCB relay (24 V) failure modes: i.e.
is it more likely that the contacts at some stage burn (when opening under
load), or does it happen more often that the coil gets it?

My gut feel was that the contacts should tend to go more often, but speaking
to the techies who normally service the units we manufacture once they're in
the field (I'm redesigning) it sounds like the coils might burn more often,
although no proper (quantitative) data is available on failures - many were
possibly misdiagnosed anyway...

Thanks
Roland

2000\03\29@155540 by Chris Eddy

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Roland;

Both failures can, of course, occur.  Both can be reduced to bare minimums,
though.

Coils are designed to run within a specific range of voltages.  If you are using
an unregulated supply, though, you can easily vary well beyond the votlage of
the coil.  Temperature rise will getcha on the coil, much faster than old age.

Contacts are not designed to handle large inductive loads without help.  If you
are breaking big currents, strongly consider carefullly designed (read
experiment) snubber or MOV circuits, and if you are using DC, use diodes and or
TVS diodes to protect the contacts.  If you keep contact overvoltage down to a
minimum, you will dramatically extend the life of the relay.  If the breakdown
voltage goes high enough across the opening contacts, you will start in an arc
over condition and if it persists to a higher votlage/current across the gap, it
becomes a plasma glow arc.  It is this second stage of breakdown that will
simply nuke your contacts in no time.

Poor man's sanity check.. Place an AM radio next to the relay, tuned to a
non-station, and listen when the relay changes state.  Try different loads, IE
resistors and inductors (another relay coil is grand).  Let us know what kind of
music you hear.

Chris Eddy
Pioneer Microsystems, Inc.

Roland Andrag wrote:

> Hello everyone,
>
> I would like to hear some comments on PCB relay (24 V) failure modes: i.e.


'Relay failure modes [OT]'
2000\04\02@144830 by Roland Andrag
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Chris, thank you very much for your long reply (sorry for the delay in my
answering).

I have opened the relays and can actually observe a (very) small spark when
the contacts open.  The relay drives a 7 VA 220 V contactor, and is rated to
7 A.

The reason that I asked the question is that in the project I'm working on
it can be fairly catastrophic if the relay fails to open and the contactor
stays in ($1000's of damage can result).  The likelyhood of this happening
and damage occuring is however fairly small.  If the probability of the coil
burning is or can be made vanishingly small <g>, two relays in series could
be used to just about rule out damage resulting from contacts sticking.
However, maintenance costs go up sharply should coil failure be prominent,
especially since servicing units involves travelling long distances to get
to them.  Relay cost is not an issue (at about $1 for an Iskra 1803, it just
doesn't feature agaist cost of other components).

Of course if the likelyhood of the contacts sticking can be made vanishingly
small, two relays in series is just a silly idea <g>.  I will look into some
snubbing circuits.  I distrust MOV's because they fail short circuit,
rendering the circuit useless. At some stage they always seem to end up
being removed from a 'broken' appliance to render it working again (for a
while, anyway).

Any more comments?

Thanks
Roland

{Original Message removed}

2000\04\02@155819 by Chris Eddy

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Roland, and others;

If you want to take the relay contact protection task even further, I found a
great book titled "noise reduction techniques in electronic systems" by Henry
Ott.  It is at borders for $90.

There is a section on relay contact protection, which details the exact failure
modes, the different circuit protection topologies, and even design equations to
determine the right component values, like R and C in a snubber.  It is a
treasure on this subject!

And on the failure protection for your system, I would strongly suggest using a
single relay, and adding an activated crobar across the AC line to snap the
breaker if the unit gets out of hand.  It could be a second relay or even a
triac.  If you like, you can even hard wire the protection, such as (if limit
switch AND AC being output FOR so much time, ACTIVATE crowbar).  Make sure the
unit lights up a light that tells your customer that the unit saved him $1000
today.  Call it the money light.  Then sell him a replacement board for $999.

Chris Eddy

Roland Andrag wrote:

> I have opened the relays and can actually observe a (very) small spark when
> the contacts open.  The relay drives a 7 VA 220 V contactor, and is rated to
> 7 A.
>
> The reason that I asked the question is that in the project I'm working on
> it can be fairly catastrophic if the relay fails to open and the contactor
> stays in ($1000's of damage can result).  The likelyhood of this happening

2000\04\02@173605 by mike

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On Sun, 2 Apr 2000 20:42:27 +0200, you wrote:

>Chris, thank you very much for your long reply (sorry for the delay in my
>answering).
>
>I have opened the relays and can actually observe a (very) small spark when
>the contacts open.  The relay drives a 7 VA 220 V contactor, and is rated to
>7 A.
>
>The reason that I asked the question is that in the project I'm working on
>it can be fairly catastrophic if the relay fails to open and the contactor
>stays in ($1000's of damage can result).  The likelyhood of this happening
>and damage occuring is however fairly small.  If the probability of the coil
>burning is or can be made vanishingly small <g>, two relays in series could
>be used to just about rule out damage resulting from contacts sticking.
>However, maintenance costs go up sharply should coil failure be prominent,
>especially since servicing units involves travelling long distances to get
>to them.  Relay cost is not an issue (at about $1 for an Iskra 1803, it just
>doesn't feature agaist cost of other components).
I believe you can get relays, designed for safety cutout applications,
with a mechanism to reduce the risk of sticking by positively forcing
the contacts open - the phrase 'positively guided contact' rings a
vague bell.

2000\04\02@231109 by Grif\ w. keith griffith

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<x-flowed>Well,,, for that much money,,, and danger?  How about just doing a conflict
monitor sort of thing like they do on traffic signal systems?  Just a logic
monitor with the known bad logic,,, if it happens,,, wham, just trigger a
big scr across the power buss.  Ugly, but simple and a stand alone unit is
easy, not depending on the main processor to be healthy to detect failures.

Oh,,, do modern relays stick due to armature magnetism?  Used to be a real
funky thing to troubleshoot,,, they worked just fine,,, just a bit slow on
release.


snip and hack
At 08:42 PM 4/2/00 +0200, you wrote:
>Chris, thank you very much for your long reply (sorry for the delay in my
>
>The reason that I asked the question is that in the project I'm working on
>it can be fairly catastrophic if the relay fails to open and the contactor
>stays in


'Grif'   N7IVS

</x-flowed>

2000\04\03@115315 by Quentin

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Roland
First use relays from a manufacturer that has been around for a couple
of years and are well represented, so that you know you can trust it.
Here's some catch 22's
Coil failing? I would say I've had the same amount of driver transistors
failing as coil failing.
MOV's? Yes they fail on long exposure (a few millisecs) of over voltage,
but what you don't see is the job they do on the other hundreds
occurrences of spikes that they've trapped. And if you get a few
millisecs or more of over voltage, then there is defiantly something
else wrong with the circuit.
Using RC filter. I've had a lot more failures with these than MOV's. And
even worse, the cap has gone short, keeping the circuit on.

My point I am trying to make is that if you are going to look for a
fault, you will find it. But here are a few suggestions:

Use two pole relays to drive the contactor. Wire both poles in series to
drive the contactor. If one contact burn, you have a very good chance
the other one would not be.

On your contactor use a spare contact to monitor the state of the
contactor. So if your PIC says the contactor must be off, but read that
its still on, the PIC can: Send a warning. Shut down the main supply via
solenoid controlled isolator or separate monitoring contactor on the
main supply.

Use FET's or TRIACs to drive the contactor. over the past few years
manufacturers of these parts strive to make the component fail open
circuit, i.e.. if it pops, it will stay open circuit. Never seen a
guarantee though. but you eliminate a driver transistor, coil and
contact. Plus you got the advantage of solid state to take care of
spikes etc.

Quentin

2000\04\03@170903 by Roland Andrag

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Quentin, thanks for the reply.. great advise, lets go through it:

> First use relays from a manufacturer that has been around for a couple
> of years and are well represented, so that you know you can trust it.
Currently using Iskra - not very well known, but been around for a while.
Fairly generic relays so second sources are available.

> Here's some catch 22's
> Coil failing? I would say I've had the same amount of driver transistors
> failing as coil failing.
> MOV's? Yes they fail on long exposure (a few millisecs) of over voltage,
> but what you don't see is the job they do on the other hundreds
> occurrences of spikes that they've trapped. And if you get a few
> millisecs or more of over voltage, then there is defiantly something
> else wrong with the circuit.
There would be (something wrong with the circuit).  I have now eliminated
any visible spark using a snubbing cap.

> Using RC filter. I've had a lot more failures with these than MOV's. And
> even worse, the cap has gone short, keeping the circuit on.
mmm. That worries me.  I have decided to go for a 600 VAC WIMA Polypropylene
cap. Should last I thought.

> My point I am trying to make is that if you are going to look for a
> fault, you will find it. But here are a few suggestions:
>
> Use two pole relays to drive the contactor. Wire both poles in series to
> drive the contactor. If one contact burn, you have a very good chance
> the other one would not be.
That is a very good idea!

> On your contactor use a spare contact to monitor the state of the
> contactor. So if your PIC says the contactor must be off, but read that
> its still on, the PIC can: Send a warning. Shut down the main supply via
> solenoid controlled isolator or separate monitoring contactor on the
> main supply.
Already using the spare contact to implement a forward/reverse interlock,
sadly.  There are other ways and means of detecting whether the contactor is
in or not, and I am torn beween the gain in reliability (failing towards the
safe side) if components are introduced to do this and the loss in
reliability should they fail.  Basically the question is whether it is in
this case worth it to sacrifice (some) reliability in 1000 units to ensure
that one unit in 1000 (10000? 100 000? numbers are not very forthcoming
around here - stats are impossible to come by) will fail towards the safe
side.  These decisions are the fun in the job, after all.

> Use FET's or TRIACs to drive the contactor. over the past few years
> manufacturers of these parts strive to make the component fail open
> circuit, i.e.. if it pops, it will stay open circuit. Never seen a
> guarantee though. but you eliminate a driver transistor, coil and
> contact. Plus you got the advantage of solid state to take care of
> spikes etc.
I would have rated solid state far more succeptable to damage by spikes
etc. - has it come this far over the last few years? (so many companies have
gone under as a result of bad reliability of solid state power electronics
such as inverters... Maybe this field has finally grown up?)


Roland

2000\04\04@043246 by Alan B Pearce

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>> Using RC filter. I've had a lot more failures with these than MOV's. And
>> even worse, the cap has gone short, keeping the circuit on.
>mmm. That worries me.  I have decided to go for a 600 VAC WIMA Polypropylene
>cap. Should last I thought.

Use a cap that is rated for infinite dI/dT - typically used in line output
stages of TV sets where the rate of change of current is extremely high. A
capacitor specified for this parameter has extremely low ESR (as far as I can
ascertain) which means it does not get localised self heating spots which cause
failure. This sort of failure is common in the polycarbonate type of capacitor
when large instantaneous currents flow.

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